Abstract The community of women with metastatic breast cancer (MBC) is growing due to increases in incidence and improved treatments for women diagnosed at earlier stages who relapse. Research that addresses this burgeoning and underserved survivor group is needed. Breast cancer (BC) treatment is associated with adverse body composition changes including increases in adiposity and decreases in lean body mass (LBM). These changes contribute to insulin resistance and inflammation which are theorized to contribute to tumor progression. Low levels of LBM are also associated with treatment side-effects and decreased cellular energy production due to mitochondrial dysfuntion which lead to fatigue, symptom burden and compromised quality of life (QOL). Lifestyle interventions with early stage BC survivors that promote positive changes in body composition changes result in enhanced QOL, reduced fatigue, and improved biomarkers of BC prognosis. To date, women with MBC have been largely excluded from these trials. Our formative work supports that women with MBC are interested in and capable of participating in a lifestyle intervention trial. Thus, based on current cancer survivorship guidelines, we propose a randomized pilot (n=40) in women with MBC to: (1) examine the feasibility and acceptability of ?Every Day Counts? - a 12 week cognitive-behavioral lifestyle intervention; (2) examine the impact of Every Day Counts on body composition changes and the associations with serum biomarkers of prognosis and survival, and patient reported outcomes; and (3) explore the effect of the lifestyle intervention on mitochondrial function and its association with symptom burden. Our multidisciplinary study applies an innovative, highly integrated physiologic model to provide a biologic basis for the impact of lifestyle behaviors on outcomes, integrating support and technology to promote intervention adherence. This study reflects ?a first step? in a research trajectory to help discern how lifestyle behaviors influence quality of life, prognosis and help make every day count for women with MBC.